TFT LCDs with touch screens have become popular in displays for industrial and retail applications, as well as in PDAs and emerging tablet PC products. The majority of touch-enabled AMLCDs (Active Matrix LCDs) are based on the resistive, capacitive or inductive touch technology. All these solutions require externally added components or screens, which would add cost and reduce the optical performance of the LCD. For example, the most commonly used and currently lowest cost approach is based on resistive touch screens, which reduce display transmittance by about 20%, increase reflectance and have a limited lifetime.
Therefore, the optical sensors are designed and integrated into the embedded touch screen during the TFT-LCD array process for solving the above-mentioned defects. In the optical sensors, the optical currents are sensed to achieve the orientation function. Since the optical sensors are compatible with the TFT-LCD array process, the production cost therefore would not be increased. Besides, it is unnecessary to use the sensing film for the touch screen. Furthermore, the optical performance of the LCD is better and the lifetime thereof is longer.
Please refer to FIG. 1, which shows the layout of an embedded touch screen in the prior art. As shown in FIG. 1, the embedded touch screen is composed of a plurality of display pixels and readout pixels, and a display pixel 11 comprises a display element and a readout pixel 12 comprises both a display element and an optical sensor element 12. Please refer to FIG. 2, which shows the equivalent circuit of the embedded touch screen of FIG. 1. As shown in FIG. 2, a 240×240 pixel TFT LCD was built with optical sensor elements interspersed among the pixels at every 8th row gate line and every 8th column data line. The display is based on an existing avionics LCD platform with an RGBW (Red, Green, Blue, White) quad pixel format. The optical sensor elements are located at every 4th white subpixel. Each optical sensor element consists of an a-Si photo TFT 331 with gate shorted to source, a storage capacitor Cst2 and a readout TFT 332, and each optical sensor element has three terminals. The SW terminal 13 determines the output of a signal, and the signal is read out from the Readout terminal 14 when the SW terminal 13 is activated. The Bias terminal 15 provides the optical sensor element a bias voltage. The SW terminal 13 is connected to gate of the readout TFT 332, the Readout terminal 14 is connected to the readout line 11, and the Bias terminal 15 is connected to a voltage source, such as a bias line of FIG. 2. The readout TFTs are connected to added readout lines running parallel to the column data lines. The photo TFTs are connected to the bias line which is common for all optical sensor components and is held at 0 to −10V
Please refer to FIG. 3(a), which shows a cross-sectional view of photo TFT 331 and readout TFT 332 in the embedded touch screen of FIG. 1. The embedded touch screen includes a upper substrate 31 with the color filter 311, an opening 312 and the black matrix 313 disposed thereon. The embedded touch screen also includes a lower substrate 33 with the photo TFT 331 and the readout TFT 332 disposed thereon, and the liquid crystal molecules are filled between the first and the second substrates 31, 33. The photo TFT 331 discharges the storage capacitor Cst2 when exposed to ambient light through the opening 312 in the black matrix 313. The optical sensor elements share the same gate line with each 8th row of display pixels. Hence, the row and column drivers are identical to those in a TFT LCD without the optical sensor array.
The photo TFTs and readout TFTs are manufactured in the same process as the addressing TFTs for the AMLCD, so that the processing is identical to that for a conventional TFT LCD without the optical sensor elements. The added optical sensor elements require only a fraction of the subpixel area and have therefore a minor impact on the pixel aperture and transmittance of the display.
The 60 readout lines for the 60×60 optical sensor elements are connected to a readout chip (not shown) with 60 channels of charge amplifiers 21 (shown in FIG. 2) as inputs. The readout chip converts the readout charges to voltages that are multiplexed to a serial signal for output to an A/D converter and subsequent image processing. The timing for the signal acquisition and data output of the readout chip is derived from the display controller (not shown) and synchronized with the display driven at a frame rate of 60 Hz.
When the display is touched, the ambient light is locally blocked by one or a few of the optical sensor elements. The surrounding optical sensors are exposed to the ambient light through the opening 312 in the black matrix 313. The storage capacitors of the surrounding optical sensor elements will be partially or completely discharged. Each time a row of optical sensor elements is read out, a current flows through the readout lines that is proportional to the integrated light exposure on the photo TFTs during the preceding frame period.
Please refer to FIGS. 3(b) and 3(c), which respectively show cross-sectional views of a display pixel and a readout pixel along cutting lines b-b′ and c-c′ of FIG. 1. In the display pixel of FIG. 3(b), a data line 333, an insulating layer 334, and a pixel electrode 335, which is electrically connected with the source of the pixel TFT, are sequentially formed on the lower substrate 33. The black matrix 313 formed on the upper substrate 31 covers the data line 333 and portions of the pixel electrode 335 to prevent light leak. In the readout pixel of FIG. 3(c), a data line 333, a readout line 336, an insulating layer 334, and a pixel electrode 335, which is electrically connected with the source of the pixel TFT, are sequentially formed on the lower substrate 33. The black matrix 313 formed on the upper substrate 31 covers the data line 333, the readout line 336, and portions of the pixel electrode 334 to prevent light leak. In comparison with the display pixel, the black matrix 313 in the readout pixel has an additionally area covering the readout line 336 and the region between the data line 333 and the readout line 335. Therefore, the aperture ratio and the optical performance of the readout pixel are affected.
Please refer to FIGS. 4(a) and 4(b), which show the equivalent circuit of a display pixel in the embedded touch screen in the prior art and the equivalent circuit of a readout pixel in the embedded touch screen in the prior art, respectively. Specially, FIG. 4(b) only shows the display element of the readout pixel, and the optical sensor element is omitted. In the display pixel of FIG. 4(a), the pixel TFT 41 has a gate connected to the nth gate line and a drain connected to the nth data line. CLC is formed between a pixel electrode, which is connected to the source of the pixel TFT 41, and a common electrode, which is connected to common voltage source. CST is formed between the pixel electrode, which is connected to the source of the pixel TFT 41, and a common line, which is also connected to common voltage source. CGS is formed between the gate and the source of the pixel TFT 41. In the readout pixel of FIG. 4(b), C′LC is formed between a pixel electrode, which is connected to the source of the pixel TFT 41, and a common electrode, which is connected to common voltage source. C′ST is formed between a pixel electrode, which is connected to the source of the pixel TFT 41, and a common line, which is also connected to common voltage source. Cread-out is formed between the pixel electrode, which is connected to the source of the pixel TFT 41, and a readout line. CGS is formed between the gate and the source of the pixel TFT 41. In comparison, C′LC and C′ST are slightly different from CLC and CST, respectively, due to the fact that some pixel area is reserved for the optical sensor element including photo TFT and the readout TFT. However, CGS in the display pixel is the same as CGS in the readout pixel.
Besides the consideration of the charge for the pixel, the cooperation with the system driving should also be considered. For the middle-to-small sized LCD, the Row-Inversion driving method with common voltage, Vcom, modulation is commonly used to reduce the output range of the data signal of a data driver. Please refer to FIGS. 5(a) and 5(b), which are schematic diagrams showing the Row-Inversion driving method with Vcom modulation at Odd Frame in the prior art and the Row-Inversion driving method with Vcom modulation at Even Frame in the prior art, respectively. In the Row-Inversion driving method with Vcom modulation respectively shown in FIGS. 5(a) and 5(b), the optical sensor is embedded in the sub-pixel of blue color, so the readout line is extended within the sub-pixel of blue color. Moreover, in FIGS. 5(a) and 5(b), the symbol “+” means that the pixel electrode voltage, Vpixel, potential is higher than the Vcom potential, while the symbol “−” means that the Vpixel potential is lower than the Vcom potential.
Please refer to FIGS. 6(a) and 6(b), which show the simulation result of Vpixel respectively in the display pixel and the readout pixel with Rown at Odd Frame (Rown+1 at Even Frame) during the pixel charge period in the prior art and the simulation result of Vpixel respectively in the display pixel and the readout pixel with Rown at Even Frame (Rown+1 at Odd Frame) during the pixel discharge period in the prior art. In terms of the charging capacity, the charging speed would slow down because of the load caused by the capacitor Cread-out. However, the input of the grey level voltage would not be affected since there is enough switch-on time for the TFT. Nevertheless, another issue would be generated due to the Vcom modulation.
Because of the Vcom modulation, Vpixel would swing with the potential of Vcom when the TFT is switched off. Since the readout pixel has the capacitor Cread-out, the voltage swing thereof is different from that of the display pixel, as shown in FIGS. 6(a) and 6(b). This would cause the inconsistency of the grey level control. The difference could be seen from the following Formula (a) and Formula (b).
                                                                        Δ                ⁢                                                                  ⁢                                  V                  normal                                            =                            ⁢                                                                                          C                      LC                                        +                                          C                      ST                                                                                                  C                      LC                                        +                                          C                      ST                                        +                                          C                      GS                                                                      ⁢                                  (                                                            V                                              com                        ,                        H                                                              -                                          V                                              com                        ,                        L                                                                              )                                                                                                        ≅                            ⁢                              (                                                      V                                          com                      ,                      H                                                        -                                      V                                          com                      ,                      L                                                                      )                                                                        (        a        )                                          Δ          ⁢                                          ⁢                      V                          read              ⁢                              -                            ⁢              out                                      =                ⁢                                                                              C                  LC                  ′                                +                                  C                  ST                  ′                                                                              C                  LC                  ′                                +                                  C                  ST                  ′                                +                                  C                                      read                    ⁢                                          -                                        ⁢                    out                                                  +                                  C                  GS                  ′                                                      ⁢                          (                                                V                                      com                    ,                    H                                                  -                                  V                                      com                    ,                    L                                                              )                                ⁢                                          ⁢                                          ⇒                                    Δ              ⁢                                                          ⁢                              V                normal                                      >                          Δ              ⁢                                                          ⁢                              V                                  read                  ⁢                                      -                                    ⁢                  out                                                                                        (        b        )            
From the above description, it is known that how to develop a pixel structure for the LCD with the embedded touch screen has become a major problem to be solved. In order to overcome the drawbacks in the prior art, an improved pixel structure for the LCD with the embedded touch screen is provided. The particular design in the present invention not only solves the problems described above, but also is easy to be implemented. Thus, the invention has the utility for the industry.